1. Field of the Invention
The present invention relates to a laser irradiation apparatus for crystallizing a semiconductor film and the like or for activating the semiconductor film after ion implantation by using a laser beam. Specifically, the present invention relates to a laser irradiation apparatus for irradiating the laser beam on a semiconductor film which is polycrystalline or similar to polycrystalline and enhancing crystallinity of the semiconductor film. Further, the present invention relates to a method of manufacturing semiconductor device using a crystalline semiconductor film formed by the laser irradiation apparatus.
2. Description of the Related Art
In recent years, the technology for forming a thin film transistor (TFT) over a substrate has made great progress and such a TFT has been developed and applied to an active matrix semiconductor device. Particularly, a TFT made by using a polycrystalline semiconductor film is superior in field-effect mobility to a conventional TFT with an amorphous semiconductor film and thus, high-speed operation becomes possible. Therefore, it is possible that control of a pixel is performed by a driver circuit that is provided over the same substrate as the pixel, instead of being performed by a driver circuit provided outside of the substrate.
Incidentally, a substrate for a semiconductor device is expected to be a glass substrate rather than a single crystalline silicon substrate in terms of the cost. However, a glass substrate is inferior in heat resistance and the shape thereof is changed easily by heating. Therefore, when a polysilicon TFT is formed over a glass substrate, in order to prevent the heat distortion, laser annealing is often performed to crystallize a semiconductor film.
The characteristic of laser annealing is that the processing time can be drastically shortened as compared with other annealing methods by radiation heating or conductive heating, and that a semiconductor substrate or a semiconductor film can be heated selectively and locally, so that the substrate is hardly damaged thermally.
It is noted that the laser annealing method described here is the technology to recrystallize an amorphous layer formed over a semiconductor substrate or a semiconductor film, or the technology to crystallize an amorphous semiconductor film formed over a substrate. Moreover, the technology to flatten or modify a surface of a semiconductor substrate or a semiconductor film is also included.
Lasers used for laser annealing are broadly classified into two sorts according to their oscillation sorts. One type is a pulsed laser sort and the other sort is a continuous wave laser sort. In recent years, it has been known that in crystallization of the semiconductor film, a crystal grain formed in the semiconductor film is larger by using a continuous wave laser than by using a pulsed laser. When the diameter of a crystal grain formed in a semiconductor film is large, the number of grain boundaries in a channel region of a TFT formed by using the semiconductor film decreases, and thereby enhancing the mobility. As a result, such semiconductor film can be applied to a device with high-performance. For this reason, continuous wave lasers have attracted attention.
Moreover, when laser annealing is performed on a semiconductor or a semiconductor film, a method for shaping a laser beam emitted from a laser into a linear shape or an elliptic shape by an optical system and scanning a beam spot (surface to be irradiated) to a surface to be irradiated is known. Since this method enables an effective irradiation of a laser beam on a substrate and enhances mass-productivity, this method is employed industrially preferably (Reference 1: Japanese Patent Laid Open No. Hei 8-195357).
In order to perform laser annealing on a semiconductor film formed over a substrate effectively, a method for shaping a spot of a laser beam emitted from a continuous wave laser into a linear shape or an elliptic shape by an optical system, and scanning the shaped beam over the substrate is employed.
In addition, a galvanometer mirror is used as a means for scanning a laser beam. The laser beam that is incident on the galvanometer mirror is deflected toward the substrate. By oscillating the galvanometer mirror to control the oscillating angle of the galvanometer mirror, the deflected laser beam can be scanned on the whole substrate. With the structure in which the laser beam can be scanned by only oscillating the galvanometer mirror, it is not necessary any more to move the substrate back and forth on a stage and the like, and thereby it becomes possible to conduct laser irradiation in a short time.
It becomes possible to focus the beam deflected by the galvanometer mirror constantly on the plane surface by converging the beam with an f-θ lens. The beam deflected by the galvanometer mirror is scanned from the edge to the center of the lens to scan the semiconductor film arranged on the plane surface. However, since the f-θ lens used as a means for converging a laser beam has different transmittance in the center and in the edge of the lens, when the f-θ lens is used for laser crystallization the energy distribution of the laser beam irradiation of the semiconductor film is not uniform and thus, the whole semiconductor film cannot be irradiated uniformly with the laser beam. When a semiconductor film is irradiated with a laser beam, however, it is required to process uniformly the semiconductor film by irradiating the laser beam uniformly. Therefore, a means for offsetting the difference in the energy distribution due to the difference of the transmittance of the lens and equalizing the irradiation energy of the laser beam on the surface to be irradiated have been required.